1. Field of the Invention
The present invention refers to switching power supplies, and especially to a method and the related circuit for protection against malfunctioning of the feedback loop in switching power supplies. More in particular it relates to a circuit for the identification of a condition of excessively high voltage at the output.
The proposed system, even if it finds more direct application in offline converters, and especially in those that use the flyback topology, can also be applied to non-insulated converters and to all the other topologies (boost, buck, forward, etc.) with suitable modifications. Of notable practical interest is the extension of the system to the boost topology, and particularly in the so-called PFC (power factor corrector) pre-regulators, that is forced switching systems whose task is to absorb from the mains an almost sinusoidal current that is in phase with the mains voltage.
2. Description of the Related Art
One of the peculiarities of the offline converters is that they nearly always present the so-called insulation barrier, that is to be constituted by two galvanically separated parts. One side so-called primary, connected to the mains by means of a rectifier bridge and comprising a switch (typically a MOSFET), whose opening and closing is suitably driven so as to regulate the power flow, and almost always the control part. One side so-called secondary, isolated from the primary and connected by means of its output gate to the load to be powered.
The galvanic insulation, requested by safety regulations, is ensured by the presence of a transformer. The transformer, constructed so as to supply adequate insulation, prescribed by the regulations, permits the passage of the energy from one side to the other for magnetic coupling, without metallic contact between them.
In such converters the output voltage typically is regulated, that is kept at a constant value upon variation of the operative conditions (input voltage, output current, temperature). This objective is achieved by resorting to feedback control: the output voltage, or more often a portion thereof, is compared with a reference voltage; their difference is suitably amplified (error signal) and elaborated by a control circuit (controller) with the purpose of determining the turn-on and turn-off times of the switch so as to zero or minimize the above-mentioned error signal.
At this point, in offline converters, a problem is presented: the output voltage is on the secondary side while the control and the MOSFET are on the primary side. It is therefore necessary to transfer the information on the output voltage from one side to the other, that is, it is necessary to cross the insulation barrier in the opposite direction and, in accordance with the safety regulations, it is necessary to guarantee at least the same insulation of the transformer. The solution to this problem consists in using another small transformer or a photocoupler.
In case of breakdown of the feedback loop, the control circuit loses the information on the output voltage and it follows that the converter delivers more power than that required by the load and the output voltage starts to increase out of control in a short time leading to the destruction of the load powered by the converter as well as the converter itself. It is thus necessary to provide for means that, in the case of breakdown of the feedback loop block the converter preventing the output voltage from going out of control.
Should some of the circuit components used for setting the output voltage at the desired value (typically of resistors) deteriorate through ageing so that a higher output voltage takes place, perhaps slowly growing in time, an eventual control system of the integrity of the feedback loop might not detect the drift of the output voltage or detect it when the voltage has reached very high values and therefore not prevent a catastrophic breakdown.
Therefore a monitoring system of the output voltage is needed that acts in parallel to the feedback loop and which, should the latter no longer control the output voltage for any reason whatsoever and the output voltage tends to increase, intervenes blocking the converter when it exceeds a preset safety limit. This function is commonly indicated with the name of OVP (over voltage protection).
In the case of pre-regulators PFC which in their most typical form use the boost topology, which is not isolated, and that therefore does not present the problem of crossing the insulation barrier, different needs, strictly linked to the safety of the application, lead to the same need. A pre-regulator PFC, in fact, generates a direct output voltage starting from the rectified and non-filtered mains voltage (rectified sinusoid). The output voltage is greater than the maximum mains peak voltage, that is typically equal to 400V. To lose control of such a high voltage would have catastrophic effects.
With reference to the offline converters, there are different known solutions to the problem outlined above, all of which can be organized into two fundamental categories: that in which the output voltage is measured directly and indirectly.
The solutions belonging to the first category provide a very precise level of intervention and can be applied to any topology of converter. However, as has already been observed for the feedback, they require the insulation barrier to be crossed.
When high precision is not required, resort is made to the indirect reading of the output voltage. In the flyback configuration this is particularly easy as during the time in which the switch is off, the output voltage is applied to the secondary winding of the transformer, therefore it is found at the terminals of all the other windings multiplied by the respective turn ratios. Usually an auxiliary winding Vaux is used, with suitable polarity, together with rectifier diode and a capacitor to derive a direct voltage Vcc destined to power the integrated control circuit during the operation of the converter. This voltage, because of what was said before, is ideally proportional to the output voltage, that is regulated, and thus stable.
If the feedback loop breakdown making the output voltage increase out of control, this increase would be reflected by a similar increase of the voltage Vcc. A comparator can thus activate the protection when the supply voltage Vcc exceeds a preset value. The system is very simple and economical but it is rather imprecise. The voltage Vcc, even though it is well stabilized upon the variation of the input voltage of the converter, is not upon the variation of the load. This is mainly caused by the parasite parameters of the transformer.
In the case of pre-regulators PFC, in addition to the obvious possibility of direct reading of the output voltage, there is a series of techniques for verifying the integrity of the feedback loop, more easily applicable to the controllers fitted with error amplifier of the transconductance type.
Direct reading permits not only, as already mentioned, maximum precision of the intervention threshold of the protection but also maximum safety (in the hypothesis, provided for by the safety regulations, of single breakdown). In fact the protection is activated whatever the cause is that generates the over-voltage, whether it be the breaking of the loop, or whether it be deterioration. Against direct reading the use of a pin is required, of the integrated circuit dedicated, and of a second resistive divider in addition to that used for the feedback loop. In addition to the increase in the number of components outside the controller, the second divider dissipates a certain quantity of power which, for as small as it is, in the systems where compliance to the most recent regulations regarding the reduction of consumption of equipment in non operative conditions is requested (for example EnergyStar, Energy2000, Blue Angel, etc.), can represent an undesired contribution that cannot be disregarded.
The techniques for verification of the integrity of the feedback loop, on the other hand, do not generally require the use of a dedicated pin or additional external components, but they do not offer a level of protection that is just as complete, as they are unable to identify movements of the regulation point due to slow deterioration of the loop. Each single time, according to the needs of the specific application, one or the other technique can be preferable.